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Repair of King luminescent / gas discharge displays, and LED replacements

That’s interesting, @A_and_C . I wonder if it is possible that the arcing could be happening “upstream” on the actual PCB tracks due to dampness/dirt etc?

United Kingdom

I think that humidity has a lot to do with the problems, my aircraft have always been hangared and I have never had any serious avionic issues with what was a king stack top to bottom for the best part of twenty years.

When I fitted the stack all the units got the display brightness adjusted on the bench before that got fitted into the aircraft and settings did match for a few years but slowly slipped out of adjustment, that was when I returned them to Simon who cleaned them and the adjustment returned to normal.

I do seem to have had less avionic trouble than the average owner and my theory for this is that first and foremost the aircraft always lives inside a dry hangar, the aircraft generally flys long legs with three and four hour trips not being unusual and finally I had an avionic cooling fan fitted from day one. I would contend the reason for my avionic reliability was first the avionics remained dry and second they remained cool even when operations in the 40c temp in Southern Europe , a secondary effect of the cooling fan may well have been to remove any moisture that was inside the units.

On a wider issue I would also contend that you pay for hangarage even if you park your aircraft outside, as the money you save from outside parking is spent on avionic repairs and other maintenance issues that would not happen if your aircraft spent most of its life in a nice dry hangar.

I have heard that quite often it is the driver chip(s) that fail. I don’t have the KDI572 schematic but the KR73 ADF shows this

and the two chips are these

and the DI-512 is here.

On a wider issue I would also contend that you pay for hangarage even if you park your aircraft outside, as the money you save from outside parking is spent on avionic repairs and other maintenance issues that would not happen if your aircraft spent most of its life in a nice dry hangar.

I agree; lots of people have reported that.

Shoreham EGKA, United Kingdom

OK, it is not the display as a known working display did exactly the same thing. Peter is correct that the driver chips often fail so brand new OEM driver chips were fitted (not the cheap Chinese copies). A transistor and a diode were replaced which often cause issues too.

Still not working! The very experienced repair shop are scratching their heads now and have never had one like this.

Any suggestions welcome or it looks like I’ll be scrapping my unit – at least there will be one more gas discharge display liberated for somebody else to use!

United Kingdom

Maybe I am naive if it comes to these displays, but @Archer-181 ’s experience looks like really awful troubleshooting.

If these were LED displays, one would apply the appropriate voltages (through an appropriate resistor, if not include in the unit) to test the display, and measure the output of the unit (and perhaps some interim stages) to establish the cause.

Can this not be done with these, or does nobody know how, so this shop resorts to trial-and-error replacement of bits?

Biggin Hill

Hi @Cobalt, the firm in question have done some excellent component repairs for me in the past and I have been pleased with their past work. The discharge display was tested in seconds by plugging in a working display they had in stock so this test took only a few minutes.

In 98%(?? guess) of the time, if the display is working then it is the driver chips. You are right that we have wasted a bit of time and the cost of the driver chips but it’s a fraction of the cost of a second hand DME so it was was worth a go.

The power supply, transmitter, receiver all seem to be working fine so it’s a weird one

United Kingdom

They are gas discharge displays and not specially high voltage.

A lot of “garden shed” avionics repair shops clean up the connections with a brush and some solvent. I think the PCB corrodes.

Any electronics person with an oscilloscope should be able to find the problem easily enough.

Shoreham EGKA, United Kingdom

So in case this helps anybody, this was the fault below as per my invoice. I am guessing there was ripple on the 90 volts power supply and they found a duff Electrolytic smoothing capacitor. This was a necessary fix but probably did not fix the display problem. Then they replaced a capacitor on what they call the modulator. I am guessing this might be the clock which runs the multiplex on the display but perhaps not. However, I am assuming it was another duff Electrolytic capacitor as the other types of capacitors are more reliable.

It is interesting that my Aircraft had been out of action for 5 months and this then failed in less than 5 hours after the downtime. Periods of disuse are when old capacitors can develop problems. Next time I will be more mindful of this. I believe it is also an issue in old King Radios. Moral of the story – check Electrolytic Capacitors first, they are cheap parts. You might not be fixing the unit yourself but it is handy knowledge to have a list of “known” faults.

Connection to bench for testing. Unit disassembled for
troubleshooting. Faulty Capacitor on Power Supply
detected and replaced. Further Troubleshooting
performed due to ripple on modulator signal. Faulty
capacitor replaced. Reassembly, modulator alignment
and complete final test acc. to Maint. Manual
__performed. Test ok_

United Kingdom

Certainly, electrolytics do dry out and generally go bad.

And that is before you get onto the “chinese fake capacitor” issue where millions of them on computer motherboards all “expanded” due to gas buildup

A good strategy for fixing “anything old and electronic” for which you don’t have the circuit diagram, is

  • check the supply voltage on known devices (e.g. +5V on 74HC chips) and fix that
  • visual check for anything broken
  • resolder all solder joints of large/heavy components (e.g. connectors)
  • replace all electrolytics
  • replace any plastic-cased power devices (thermal cycling kills these)

Don’t use unleaded solder; it is crap. Use the proper leaded stuff which is toxic, carcinogenic and produces birth defects Well, there is some Japanese unleaded stuff – Almit – which works (for hand soldering) but we pay £70 per 0.5kg for it…

Shoreham EGKA, United Kingdom

So more information as I just spoke to the guy who repaired it. It was the first Electrolytic on the high voltage power supply which was causing the display issue. The second one was actually in the RF section and this showed up during the bench test. It might help people with duff displays in the future as a decent quality capacitor probably costs less than £2

United Kingdom
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